Turbomachine having a compensating piston

ABSTRACT

A turbomachine including a compensating piston that is sealed by means of a seal with respect to a stator is provided. Conventional compensating pistons consume significant amounts of process fluid, which can cause a considerable loss of efficiency. The turbomachine provides the solution in that the stator partially carrying the seal is designed to be axially moveable in the housing such that the permeability of the seal can be changed in this way.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2010/050643, filed Jan. 20, 2010 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2009 007 713.8 DE filed Feb. 5, 2009. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention refers to a turbomachine with a balance piston, which balance piston has at least one shaft step on a rotor which extends along a machine axis, and a seal, which seal seals a gap between the rotor and a stator so that during operation a cavity which is adjacent to a first side of the seal can have a pressure which, by a pressure differential across the seal, differs from that on the other side of the seal, wherein the shaft step is adjacent to the cavity.

BACKGROUND OF INVENTION

In turbomachines, for example steam turbines, gas turbines or compressors, especially axial and radial compressors, a pressure difference across the impeller is created as a result of the expansion or compression of the process fluid, which pressure difference, acting on the adjacent surfaces of the rotor, brings about an axial thrust of said rotor. This axial thrust is dependent upon the operating conditions of the turbomachine and can also act in different directions depending upon the operating conditions. Depending upon the differential pressure and the surface conditions of the rotor, the axial thrust is a substantial force which, with an exclusive counter force by means of a thrust bearing, gives rise to a considerable overall size of said thrust bearing. Therefore, turbomachines regularly have a so-called balance piston which serves for thrust compensation. This is regularly designed in such a way that a lower thrust remains in a specified direction under all operating conditions, so that no load change reactions take place and the rotor is not moved back and forth within a possible axial clearance.

Balance pistons regularly have at least one shaft step, wherein they are frequently designed as a radially extending disk on the rotor and a shaft seal which is adjacent to the at least one shaft step seals the rotor to a stator. The most frequent type of construction is the mentioned disk type, so that on the two flanks or shaft steps, which are defined by the disk, different pressures create the necessary axial thrust for compensation. Formed adjacent to the shaft step or the shaft seal and the stator is a cavity in which prevails a first pressure of a process fluid, which differs from a second pressure on the other side of the cavity by a differential pressure across the shaft seal. Acting upon the shaft step, the first pressure, with the surface of the shaft step, results in a force which, correspondingly dimensioned, at least partially compensates the axial thrust and consequently facilitates the task of the thrust bearing of the shaft by the amount of the compensating force.

In order to permanently maintain a pressure difference across the shaft seal of the balance piston, it is necessary to feed process fluid through a balance line to the cavity or to extract process fluid from this, since conventional shaft seals always have a significant leakage. The quantity of process fluid which flows through the balance line can have a significant value and can considerably lower the efficiency. This effect becomes worse if the shaft seal on the balance piston is worn as a result of erosion or rubbing on the stator, and consequently slackness increases.

A system for balancing axial thrust, with a balance piston which is acted upon by a pressure for thrust compensation only in case of requirement, is already known from European patent EP 1 418 341 B1, wherein during normal operation the thrust compensation is carried out by means of the applicable shaft seals at the penetrations through the casing, so that during normal operation supplying the balance piston with process fluid is not required.

SUMMARY OF INVENTION

Starting from the aforesaid problems, the invention has been based upon the object of reducing the loss of process fluid which arises on the balance piston and thus improving the efficiency of the machine.

For achieving the object, it is proposed according to the invention that the stator is designed with guided movement capability in such a way that the gap can be increased and decreased. On account of the design of the stator with guided movement capability, a gap width which is adapted to the current operating conditions of the turbomachine can be established, the gap width being smaller than that which is required under other operating conditions, so that the differential amount of the quantity of process fluid which flows through the balance piston is saved for the benefit of efficiency improvement.

An advantageous development of the invention provides that the stator by one surface is adjacent to the cavity and is designed in such a way that, depending upon the pressure in the cavity, it is displaced in such a way that the gap increases or decreases. This type of design opens up the possibility of realizing a gap width of the seal on the balance piston which is automatically adapted to the pressure in the cavity without additional actuators, which move the stator, or without a complicated control system.

The stator can advantageously have an elastic element which, depending upon the position within the limits of the guided, preferably axial movability, creates a restoring force which counteracts the pressure in the cavity. Depending upon the pressure conditions in the cavity and on the opposite side of the balance piston, the force from the elastic element acts upon the stator in the same or opposite direction of the force from the pressure in the cavity upon the stator.

This elastic element can be expediently designed in such a way that, for each pressure in the cavity, a gap which matches the operating state associated with this pressure is established. The widening or decreasing of the gap can be produced in a particularly simple manner by the annulus of the gap which extends circumferentially around the machine axis not extending parallel to the machine axis in the axial direction, but extending obliquely to it, and by the movability of the stator being guided in the axial direction. This design has the advantage that the gap can be varied without a variation of the involved components with regard to their circumferential dimensions. In this case, the surfaces of the rotor and of the stator which delimit the gap are designed as a cone in each case. The cone of the rotor and of the stator can have different cone angles to each other, which, in specific operating ranges, can have positive effects upon rotor stability.

A balance line to the cavity for feeding or draining process fluid in order to establish a pressure in the cavity, which is advantageous for the operating state, proves to be particularly expedient. On the other side of the cavity, this balance line opens into a pressure reservoir of the process fluid or into a pressure sink or suction line, and provision is particularly advantageously made in the balance line for a control valve by means of which a defined pressure difference across the control valve can be applied. The control valve enables the controlling of the pressure in the cavity by means of controlling the inflowing or outflowing quantity of process fluid.

With the device according to the invention, the efficiency of the turbomachine, especially of a compressor, can be particularly advantageously increased, after a starting process of the turbomachine, by the quantity of process fluid which flows through the balance line being reduced by means of the control valve—during steady state operation—to the extent that it allows the remaining clearance of the seal on the balance piston. The elastic element, the stator and the inclined position of the seal are expediently matched to one another in such a way that a reduction of the quantity of process fluid which flows through the balance line and the seal by means of the control valve results in a decrease of the gap width of the seal. Vice versa, it makes sense if, with an increase of the quantity of process fluid which flows through the balance line, the gap width in the seal on the balance piston also increases.

In the following text, the invention is explained based on a special exemplary embodiment with reference to a drawing. The exemplary embodiment serves simply for the better understanding of the invention, wherein further embodiment possibilities of the invention are opened up to the person skilled in the art from the present description and especially from any combination of the features which are mentioned in the patent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 shows a schematic view of the turbomachine according to the invention with balance piston, in a longitudinal section.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a compressor or a turbomachine T with a casing CAS and a rotor R which extends along a machine axis X. The rotor R has an impeller IMP—being the rotor wheel of a centrifugal compressor in this case—by means of which a process fluid PF, by a rotation at the speed n of the rotor R, is compressed from a first pressure pl to a higher second pressure p2. For this purpose, the process fluid PF enters the casing CAS through an inflow IN and leaves the casing through an outflow EX. Before the process fluid PF leaves the casing CAS through the outflow EX, it accumulates partially in a pressure chamber PCH. Adjacent to the pressure chamber, the rotor R is provided with a balance piston BP. The balance piston BP basically has a disk shape which, with the two-dimensional extent, extends perpendicularly to the machine axis X, that is to say correspondingly has a first shaft step RS1, which in this case is adjacent to the pressure chamber PCH, and a second shaft step RS2, which is adjacent to a cavity CH.

A gap between the balance piston BP and a stator ST, which is capable of guided axial movement in the casing CAS, is sealed by means of a seal SS which is designed as a shaft seal—in this case as a labyrinth seal—so that a pressure difference ensues between the pressure p2 in the pressure chamber PCH and the pressure p3 in the cavity CH, the pressure chamber and cavity being sealed in relation to each other. So that this pressure difference ΔPBD is maintained, despite the slackness of the seal SS, provision is made for a balance line BPL. For draining overflowing process fluid PF which flows from the pressure chamber PCH to the cavity CH via the seal SS, the balance line BPL is connected to a pressure sink which in this case is the inlet IN into the casing CAS.

Provision can also be made for the case in which the balance line is connected to a pressure reservoir if the pressure difference ΔPBD across the balance piston BP is oriented in reverse (in the case of a different orientation of the impeller, for example).

The balance line BPL is provided with a control valve CF via which a pressure difference ΔPCV is created depending on the position of the control valve CF.

A part of the seal SS is attached to the stator ST which is designed with guided axial movement capability, wherein the gap GP to be sealed extends obliquely to the machine axis X. In this way, the spatial geometry which is defined by the gap GP forms a cone which extends concentrically around the machine axis X in the circumferential direction. Since the turbomachine T of this exemplary embodiment is a centrifugal compressor or compressor, and the balance piston BP is located on the side of the higher pressure of the impeller IMP so that the pressure chamber PCH has a higher pressure in comparison to the cavity CH, it is expedient if a displacement of the stator ST in the direction of the chamber with the lower pressure (in this case the cavity CH or p3) brings about an increase of the gap GP or of the gap width H, and as a result therefore creates greater slackness of the seal SS. The stator ST is pretensioned by means of an elastic element EE in such a way that the restoring force of the elastic element EE acts in the direction of a decreasing of the gap width H. If during steady state operation, with initially constant gap width H, the control valve CV is throttled, the pressure p3 is increased and a new equilibrium state of the arrangement is established in such a way that the gap width H decreases as a result of the greater thrust upon the stator ST in the direction of the pressure chamber PCH. According to the invention, by means of the control valve position CV, the axial thrust from the balance piston BP can be adjusted and the consumption of process fluid PF through the balance line can be reduced. 

1.-6. (canceled)
 7. A turbomachine, comprising: a balance piston, wherein the balance piston includes a shaft step on a rotor, which extends along a machine axis, and a seal seals a gap between the rotor and a stator so that a cavity, which is adjacent to a first side of the seal and is filled with process fluid during operation may have a pressure which by a differential pressure across the seal, differs from that on the other side of the seal, wherein the shaft step is adjacent to the cavity, wherein the stator is designed with guided movement capability in such a way that the gap may be increased and decreased, and wherein a balance line opens into the cavity, by means of which process fluid may be fed to the cavity or may be drained from the cavity.
 8. The turbomachine as claimed in claim 7, wherein a surface of the stator is adjacent to the cavity and is designed in such a way that, depending upon the pressure in the cavity, it is displaced in such a way that the gap increases or decreases.
 9. The turbomachine as claimed in claims 7, wherein the stator comprises an elastic element which, depending upon a position of the stator, creates a restoring force.
 10. The turbomachine as claimed in claim 7, wherein the gap does not extend parallel to the machine axis.
 11. The turbomachine as claimed in claim 9, wherein a control valve is in the balance line, by means of which feeding the process fluid or draining of process fluid may be adjusted.
 12. The turbomachine as claimed in claim 11, wherein the position of the stator may be adjusted in axial movability by means of the control valve during operation. 